The present invention relates to golf balls. More particularly, the present invention relates to a golf ball having a cover consisting of a plurality of layers having physical properties that vary as a function of ball thickness.
Golf balls generally consist of an internal core surrounded by a cover. The core is typically either a solid rubber core or a wound core. FIG. 1 shows a two-piece ball construction having a rubber core 20 surrounded by a relatively thick thermoplastic cover 22 with dimples 24. The cover 22 is often mounted over the core 20 by injection-molding or by heat-molding together two "half cups" that are produced by injection.
Generally, the core consists of a soft material that resiliently deforms upon impact with a golf club. As it resumes its shape, the core propels the ball from the club face. The core is thus the "engine" which largely determines the distance the ball travels upon being struck with a club. On the other hand, the cover is hard relative to the core. Because the outer portion of the cover contacts the club face, the cover determines the feel of the ball at impact. The softness of the cover also determines the ball spin rate. Generally, if the cover is soft, the spin rate of the ball increases and improves the feeling of the ball when struck. However, the drawback of a soft cover is a significant loss in the ball distance. On the other hand, when the cover is hard, the ball travels farther but the spin rate reduces so that the ball is more difficult to control with the short clubs, such as for example, when the ball is manufactured of high flexural modulus ionomers. Thus, with the current two-piece construction a manufacturer must choose between acceptable distance characteristics and acceptable feel and control characteristics.
Manufacturers have attempted to make golf balls with multiple layers in order to resolve the apparent contradictory relationship between the distance characteristics and the control and feel characteristics of a golf ball. FIG. 2 illustrates a three-piece golf ball that consists of a solid core 20, a mantle 21 of thermoplastic material, and a cover 22 manufactured of a different thermoplastic material. A three-piece golf ball generally allows a manufacturer more latitude in varying the physical and dimensional properties of the ball. A conventional cover generally includes only one or two layers having thicknesses between 0.050 and 0.095 inches so that the cover is resistant to cutting and abrasion while still providing the ball with sufficient rebound properties.
U.S. Pat. No. 5,253,871 to Viollaz discloses a three-piece golf ball having an elastomer core, a mantle, and a cover. In the Viollaz patent, the cover is a single layer of a predetermined hardness, with a thickness of at least 0.035 inches. The mantle has a hardness different from that of the cover. The mantle thickness ranges between 0.039 to 0.118 inches. Although the three-piece construction of Viollaz allows a manufacturer more latitude in adapting the ball to the various conditions of a game, it also has certain drawbacks, particularly the hard and thick cover which generally does not provide sufficient spin upon impact.
U.S. Pat. No. 5,439,227 to Egashira also discloses a three-piece ball. The ball has a soft mantle and a hard cover. The cover thickness ranges between 0.058 and 0.106 inches. The ball construction disclosed by Egashira has the same drawbacks described above with respect to the previous reference.
U.S. Pat. No. 5,184,828 to Kim discloses a golf ball having a double core of variable hardness. According to Kim, the variable distribution of hardness allows a high energy to accumulate in the region of differing hardness. However, because of the relatively large distance between the core and the cover, the hardness variation only affects the core of the ball and not the surface of the ball when the ball is struck. Consequently, when the ball is struck with a short iron, which provides relatively little deflection to the cover, the core provides little or no significant effect on the ball spin.
U.S. Pat. No. 4,919,434 to Saito discloses a two-piece golf ball consisting of a solid core and a cover having a thickness of 0.4 to 2.2 mm thick. The cover consists of a 0.1 to 2 mm inner layer and 0.1 to 1.5 mm thick outer layer enclosing the inner layer. Both the inner layer and outer layer are made of thermoplastic resins, with the inner layer being a soft thermoplastic resin. The outer layer is formed of a harder thermoplastic resin having a flexural modulus of 2000-5000 Kg/cm.sup.2. However, Applicant has observed that only two graduations of hardness around the core does not sufficiently enhance the characteristics of the ball, particularly the cover hardness. Thus, Saito does not produce a cover that is optimized for each club in a set.
According to USGA rules, a player can have a maximum of fourteen clubs in a bag. A typical set of clubs includes three woods of different lofts, 10 to 12 irons, and one putter. Depending on various factors, including the type of club and the head speed, the ball deforms differently against the club face and experiences differing momentum upon impact. In particular, the percentage of inward deformation of the ball reduces gradually from the driver or other long clubs to the shorter clubs. Furthermore, except for the putter, the momentum of the ball increases in the direction of deformation, resulting mainly from the increase in the loft angle of the club face. A set of clubs is intentionally arranged to provide increased control over the ball as the club length shortens.
The balls of the prior art, such as the ball disclosed by Saito, are not configured to perform optimally with each club in a set, or at least each of the principle clubs in the set. For example, in the Saito patent, the ball cover must have a minimum thickness to protect the core and to provide proper resistance against abrasion and cuts. If one of the layers is made thin, the thickness of the other layer must be increased in order to preserve the minimum thickness. The ball properties, particularly hardness, are controlled by the thickest layer. For instance, if the soft inner layer is made thin to provide the ball with increased distance, the harder outer layer must be made thicker so that the ball has a hard feel and poor control. On the other hand, if the hard outer layer is made thin to improve feel, the soft inner layer must be made thicker, which improves feel but reduces distance. Thus, it is not possible with only two layers to optimize both ball distance for the long clubs and ball control for the short clubs while also maintaining the minimum cover thickness for durability.
Because the ball deformation gradually reduces from the driver to the short clubs, there is a need to precisely adjust the ball hardness so that the ball reacts differently depending on the amount of deformation that the ball experiences at impact. That is, ball performance would be optimized if the properties of the ball as a function of inward distance were particularly suited to the various types of clubs. For example, the hardness of the outer layers of the cover may be optimized for putters depending on whether a golfer prefers a soft or hard putting feel. Likewise, the hardness of the intermediate layers could be optimized for pitching wedges, which deform the ball slightly inward into the cover. The hardness characteristics of the innermost layers should be optimized for the wood-type clubs, which produce the greatest inward deformation, often deforming the center of the ball.
None of the prior patents have sufficiently dealt with the need to adapt the structure of the ball to the various degrees of deformation that occur upon impact with different golf clubs.
There is therefore a need for a multi-layer golf ball having a structure that is adapted to respond optimally for each club used during a game. Such a golf ball should have equal or better flight performance characteristics than a conventional ball when struck by a driver or any similar long club, as well as when struck by a middle iron and a short iron. The ball should also be designed to impart a soft or a hard feel when hit by a putter, depending upon the golfer's particular preferences.